In disk drives, defects on the media surface can cause the read channel to repeatedly detect incorrect data (hard errors). Very large defects may result in hard errors that are too long for the disk drive ECC algorithm to detect. Defect scans are used in the manufacturing process to flag those sectors with large defects so they are excluded from use during normal drive operation. The defect scan involves two main steps:
1. Write entire media surface with a high frequency repeating patterns;
2. Read back each sector and check for unusual changes in head signal amplitude.
A high frequency pattern is written to maximize the probability of actually writing a transition on a small defect. If a transition is written on a defect, the resulting magnetic head amplitude increases or decreases based on the type of defect. A decrease in magnetic material on the media correspondingly decreases the amplitude of the read back signal (resulting in a localized read back signal drop-out) and an increase in magnetic material on the media correspondingly increases the amplitude of the read back signal (resulting in a localized read back signal drop-in).
Based upon the number of detected defects, it may be determined whether the disk drive is useable or not. The disk drive may fail the manufacturing process when too many defects are detected. Conventionally, if the disk drive is determined to be usable given the number and severity of the detected defects, a predetermined space around the detected defect is designated as a margin that becomes unavailable for user data. However, experience has shown that a detected defect may spread or “grow” from its original position during subsequent use of the disk drive—that is, after the drive has been shipped to the customer. Such defects are commonly known as growing thermal asperity (TA) defects. Conventionally, media defects are detected, margined, and mapped during the manufacturing process. Consequently, when a drive leaves the factory, shipped to the customer and put to use in the field, it is assumed that the probability of finding any new defects is low. Consequently, the capacity to accommodate new defects, such as growing TA defects, in the field is limited.
While TAs are primarily margined out during the manufacturing process, conventional drives do not provide any mechanism for margining out a newly found TA and relocating the data previously stored in the margined out sectors after the drive is shipped and put to use. Due to TA's varying heights and material used to construct the media (AlOx in particular), a thermal asperity site must be margined extensively (in the order of +/−2 um, or about 80 tracks in a 350K tracks per inch (TPI) hard disk drive design) to avoid collisions with the magnetic head while it is track following in the vicinity. While the margining process is relatively straightforward during manufacturing, it is virtually impossible to carry out margining of a newly-found TA defect after the drive is formatted. Legacy defense methods against grown/missed defects in the field, i.e., relocation, are limited in their capacity and capability and are not is practical to implement for newly-found TAs.
A relocation event is designed to move a defective sector from its previously designated location to a new location. Relocation occurs during a normal write command to a sector previously marked, such as by a TARE (Transparent Automatic Relocation Event) entry, for example. To ensure data integrity, the action involves multiple writes to the new location followed by repeated reads to verify the data. Since this activity takes a long time to complete, relocations are generally done one sector at a time to minimize long write command completion time. As a result, the command time constraints make large scale relocation of many contiguous sectors impractical, if not cost prohibitive, from a timing point of view.